1. Field of the Invention
The present invention relates to a drive control apparatus for a motor, and more particularly, to a drive control apparatus capable of switching the rotation of a motor from high-speed rotation to low-speed rotation or stop of the rotation in a short time.
2. Description of the Related Art
A well-known type of a spindle motor for driving a magnetic disk employs the Frequency Generator (FG) servo method and the Phase Locked Loop (PLL) servo method together. The FG servo method detects the rotation speed of a motor as frequency by an FG, converts the frequency to a DC voltage, compares the converted voltage with a reference voltage, and controls a voltage applied to an armature coil in accordance with a differential output obtained by the comparison. On the other hand, the PLL servo method detects the rotation speed of a motor by an FG, compares a phase of a rotation frequency signal whose waveform is shaped and a phase of a reference frequency of a reference oscillator, and controls the rotation speed of the motor to be constant by making both the phases agree with each other. The combination of the FG servo method and the PLL servo method makes it possible to prevent incorrect synchronization and to obtain high stability of the rotation precision.
In general, in order to control the speed of the motor, two kinds of state times, that is, acceleration and free run, are controlled by a control means so as to increase or decrease the speed. For example, even in the case of constant-speed control, natural deceleration, that is, deceleration by the control of the time of free run is carried out.
In the above speed control, however, the increase of speed is sufficiently possible, while there is a problem in deceleration since no decelerating force sufficiently acts. In other words, it is necessary to set the state time of free run long in deceleration, and therefore prompt speed change cannot be performed because of the time limit.
Furthermore, in such a kind of motor speed control, the motor is stopped by generating counter torque after rated rotation and decelerating the motor in a short time by the action of braking force arising from the counter torque.
However, depending upon a driving IC (semiconductor circuit) to be used, the motor can be stopped in a short time only after rated rotation, while counter torque does not act in response to a stop command before the rated rotation and the motor cannot be stopped in a short time.